you may be able to get your hips back a lil more, but since you're doing high bar the knees will go more forward than in low bar, so from the angle you got it doesn't look so bad to me. Definitely a lil back rounding at the bottom but it's not horrible. Could be for a number of reasons, imbalance/flexibility/weakness in core or hips etc. Aside from trying low bar, I have always had great success with clients by putting something at the correct height behind them to either sit down on or just touch when first learning. This helps for a few reasons: 1. it's a safety net when experimenting with pushing the hips back, where in case they push back too far, they can just sit down instead of falling on their ass and dropping the weight, I find it allows for people to get more comfortable with sitting back more. 2. I have found that for clients struggling with keeping their back straight, it helps for them to completely sit down at the bottom of the squat, and then before standing back up readjusting their back by arching as hard as they can before standing back up, often times they can get a better back position on the way up then they did on the way down. This allows them to really fight for the straight back position and get some good dynamic and squat-specific stretching in. From this they progress to putting less pressure on the box (instead of totally sitting down) until they can just simply touch the box with their butt and not sit down. At that point ya just get rid of the box and the back position is usually better. 3. Lastly, for some it just helps as a reference point so they have the same depth each time, and in this case they would also only be touching the box, not sitting.
Just to be clear, I'm not trying to recommend that beginners do box squats as opposed to regular squats. I'm just saying a box can be used as a learning tool, and with light weight I should add. The goal is to get off the box as soon as possible and never to have reliance on it